Terminology

Boot:
These are what the different screens you can boot in to look like:

S-ON vs S-OFF
S-ON means "Security is on". This means your phone is not rooted. You can check your security at the HBOOT screen. In contrast, S-OFF means "Security is off". This means your phone is rooted.

Others
A kernel is the file that manages system resources. Most people install a new kernel to increase their battery life.

A ROM is an edited operating system you can install on your phone.

Nandroid backups are complete backups of the data on your phone. They are used to restore the system in case you break it.